Catalysts comprising coprecipitated composites of alumina with oxides of one or more of the iron group metals (iron, cobalt and nickel) are very well known in the art. However, conventional methods for preparing such catalysts usually involve adding to an acidic solution of the desired metal salts a highly alkaline ionic precipitant such as ammonium hydroxide, potassium hydroxide, sodium carbonate, etc. This technique always results in non-homogeneous coprecipitation because the respective metal hydroxides precipitate at different instantaneous pH's; the initial distribution of the precipitant in the aqueous medium is non-homogeneous, with the result that precipitation occurs non-homogeneously under widely differing pH conditions throughout the body of the liquid. Under these conditions it is obvious that a uniform coprecipitate of aluminum hydroxide and the active metal hydroxide will not be obtained, and there will not be a high degree of dispersion of the active metal in the coprecipitate. The resulting compositions are generally deficient in thermal stability, due presumably to the tendency of the unbound and heterogeneously distributed active metal to migrate and form larger agglomerates at high temperatures.
It is known (as described for example in my U.S. Pat. No. 3,147,227) that individual hydrous metal oxides and/or phosphates can be precipitated as hydrogels by the slow hydrolysis of urea. This is homogeneous precipitation because the precipitating agent (ammonia) is released slowly and uniformly throughout the body of the solution. However, it would hardly be expected that a uniform coprecipitation of two or more hydrous metal oxides could be obtained by this method because of the large variation in pH at which precipitation of metal hydroxides occur. Thus, zirconium hydroxide begins to precipitate at pH 2.0, aluminum hydroxide at 4.1, nickel hydroxide at 6.7, and magnesium hydroxide at 10.5.
I have now discovered however that a delayed precipitant such as urea, which by hydrolysis liberates ammonia and carbon dioxide uniformly throughout an aqueous medium, can be used to effect a homogeneous coprecipitation of aluminum hydroxide and an incompletely identified basic compound of one or more of the iron group metals when the latter is initially present in solution as a divalent metal salt. This coprecipitation begins to take place at about pH 4.0-4.5, and continues substantially homogeneously until completed at a pH in the range of about 6.0-7.5. The explanation for this phenomenon is not understood completely or with certainty, but it would appear that the ionic divalent metal may precipitate as some form of basic carbonate, beginning at substantially the same pH at which aluminum hydroxide begins to precipitate. X-ray analyses of the coprecipitated, dried nickel-alumina composites have given diffraction patterns very similar, but not identical, to that of basic nickel carbonate, NiCO.sub.3.2Ni(OH).sub.2.4H.sub.2 O.
After drying, calcining, and reducing, the coprecipitated composites are found to display a remarkable degree of thermal stability, as well as high activity in chemical conversions wherein fluid reactants are converted to chemically different products at elevated temperatures under reducing conditions. Surprisingly, these results are obtained even though the surface area of the active metal in the final catalyst is quite low, generally below 18 m.sup.2 /g, even for catalysts containing as much as 60 weight-percent of active metal oxide.
The foregoing surprising results are not merely the function of a precipitating medium containing both hydroxide and carbonate ions. U.S. Pat. No. 3,320,182 to Taylor et al discloses a coprecipitated nickel-alumina catalyst, employing as the precipitant ammonium bicarbonate. As will be shown hereinafter, catalysts prepared by this method are definitely inferior in thermal stability, compared to the catalysts of this invention.
The remarkable properties of the present catalysts are not however attributable solely to the homogeneous precipitation technique; homogeneous coprecipitation is required. The urea hydrolysis method can be applied separately to an aluminum salt solution, and to one of the iron group metal salt solutions to obtain homogeneously but separately precipitated components. But upon admixture of these separately precipitated components, the finished catalysts obtained therefrom are found to be much inferior in thermal stability, and the alumina surface area after calcination is very low. It would thus appear that homogeneous coprecipitation from the same solution results in some chemical and/or physical interaction between the two components, which not only produces superior thermal stability, but stabilizes the surface area of the alumina component.
Although it is not possible to account with certainty for the thermal stability of the catalysts of this invention, it is believed attributable at least in part to the presence therein of a "reservoir" of inactive divalent metal-aluminate which during use under reducing conditions slowly generates fresh active metal surface, replacing the active surface area being lost through agglomeration. Electron diffraction studies have detected nickel aluminate crystallites in the nickel-alumina catalysts prepared herein, and its presence could account for the relatively low surface area of active metal found in the freshly reduced catalysts.